Vacuum bags and composite layup tools are used in various manufacturing and industrial processes such as the fabrication of composite structures and bonding of parts. For example, in the aerospace industry, without limitation, vacuum bags may be used to bond heat shield tiles to a metal backing Vacuum bags may also be used in vacuum bag molding wherein a flexible bag formed from a polymer sheet such as Nylon® is placed over a part pre-form and sealed along a mold flange. A vacuum drawn on the bag eliminates entrapped air and excess resin resulting in compaction of the pre-form. In addition, vacuum bags may be used to consolidate prepreg composite structures such as during curing of a composite structure.
Similarly, vacuum assisted resin transfer molding (VARTM) may employ a vacuum bag to cover a pre-form or dry fabric on one-sided tooling. Air is evacuated and liquid resin from an external reservoir is drawn into the bag which is infused into the pre-form. Any leaks in the vacuum bag may allow air to enter and form bubbles in the resin matrix resulting in an unacceptable amount of porosity in the matrix. Leaks in the vacuum bag may be relatively small and therefore difficult to observe by an operator. While various gas leak detection techniques are known, they are not always reliable and may not detect leaks quickly enough to be useful in repairing leaks while molding processes are underway.
Tools for manufacturing composite articles may also occasionally include leaks which may affect the quality of the cured composite structure. For example, composite tools for relatively large structures may be formed as a welded structure which may then be machined into the final shape of the tool. Leaks may be present in the weld and which may go undetected during the initial usage of the tool. Composite structures formed on such tools may include undesirable effects as a result of exposure of the composite layup to air leaks in the tool. Composite structures formed on carbon fiber tools may likewise include undesirable effects due to air leaks in the tool. Such leaks may occur as a result of micro-cracking due to repeated thermal cycling as the tool is heated to a high temperature for curing a composite part after which the tool may be cooled to room temperature. Such micro-cracking may result from the difference in the coefficient of thermal expansion of the carbon fibers relative to the lower coefficient of thermal expansion of the resin and which may result in shrinking of the resin at a higher rate than the carbon fibers during cool down.
Prior art attempts at detecting leaks in tools include a system where a three-ply layup of fiberglass pre-impregnated material is applied to the tool. The fiberglass part is vacuum-bagged on the tool and cured. The cured fiberglass part is then inspected for indications of tool leaks which may appear as air bubbles in the cured fiberglass part. An autoclave may be used for curing the fiberglass part. Unfortunately, if the tool maker lacks an autoclave, the tool must be shipped to the production facility without first checking for leaks in the tool and repairing any leaks that may exist. Furthermore, checking the tool for leaks using the three-ply test requires the use of an autoclave at the production facility and the availability of skilled labor to lay up and cure the part. During the curing of the fiberglass part, the autoclave is unavailable for production use. In addition, the costs for material required for the three-ply test may be significant for relatively large tools.
Another method of detecting leaks in tools includes the use of a helium detector wherein a helium emitter is moved along the tool when placed under vacuum. When helium is drawn into a leak, the detector is activated in order to indicate the location of the leak. Unfortunately, the accuracy of helium detectors may be affected by certain bagging films which may have a permeability that is large enough to absorb helium. As a result, the helium detector may provide false indications during leak checking.
A further method of detecting leaks in tools comprises a vacuum drop check which is typically performed at the tool manufacturer and wherein a vacuum bag is applied to the tool for a predetermined period of time after which the vacuum is withdrawn. The vacuum pressure is then monitored to determine whether an increase in pressure within the vacuum bag exceeds a predetermined limit. If the tool passes the vacuum drop check, the tool is then sent to the production facility where the 3-ply test is performed. If the tool fails the 3-ply test, then the tool is shipped back to the tool manufacturer for repair.
One drawback associated with the vacuum drop check is that although the tool may meet vacuum integrity requirements during the initial vacuum check, leaks may develop during subsequent use of the tool to an extent that may affect the fabrication of composite parts. In addition, although the vacuum integrity test may provide a means to indicate the presence of a leak, the vacuum integrity test may lack the capability to allow for identifying the location of leaks on the tool. Another drawback associated with the vacuum drop check is that the vacuum drop check may not provide an indication as to whether the leak is in the tool, in the vacuum bag or in the seal that seals the vacuum bag to the tool.
As can be seen, there exists a need in the art for a system and method for detecting leaks in tools and vacuum bags that provides a reliable indication of the location of a leak. Furthermore, there exists a need in the art for a system and method for detecting leaks in tools and vacuum bags which is accurate and which provides a means for rapid detection of the location of leaks in the tool and/or vacuum bag without the use of complex, external leak detection equipment.